DNA 1240H-2 Water-Surface Bursts. Data indicate that slurry-type radioactive particles will deposit on weather decks of ships caught in the fallout resulting from water-surface shots, and the deposited particles generally are not visible. On the unwashed areas of test ships, a dose increase of 40% to 50% was recorded during the 24-hr period following cessation of fallout from 2 test shots. The increase indicated that although it was invisible, deposited activity was present. The wash- down countermeasure has been effective in removing slurry deposit, and has reduced doses on washed weather decks to about 10% of the unwashed deck doses. The interaction of a ship's structure with radiations from the de- posited material serves to attenuate the gamma rays, the amount of attenuation being dependent on the thickness and density of the structural shielding. The effectiveness of the shielding is indicated in terms of the shielding factor, which is the dimensionless ratio of the below-decks dose or dose-rate to that at 3 ft above the weather deck. At below-decks locations where deck plating served as part of the shield, test data from target ships indicated that doses were 10% to 20% of weather deck values. Test data showed that the alumimm superstructure also to some extent attenuated the gamma radiations from deck-deposited activity; depending on the location of the exposure point, doses were reduced to 10% to 60% of weather deck doses. Underwater Bursts. A burst at mid-depth in comparatively shallow water, such as Shot Baker, Operation Crossroads, may be expected to produce a large base surge, as well as fallout. Records are not available to indicate whether the deposited activity from Shot Baker was visible; but it is expected that for a burst of this type, some bottom material (which would be visible) would be included in the fali- out. It was estimated that first-hour doses ranging from 3800 r to 140 r resulted on the weather decks of ships from 500 yd to 2000 yd from surface zero, respectively. Below-decks dose records, of dubious reliability, indicate shielding factors from 0.25 to 0.025 for various locations. Deeper underwater test shots produced base surges; however, no visible fallout occurred, and data indicated negligible deposited activity on the target ships. However, very small (less than 1 micron) radioactive particles were found in some of the activity samplers at Operation Hardtack. Particles, such as those in the samplers, may have been deposited on the weather decks and rapidly removed by the washdown systems operating on the target ships, since very little dose was accumulated on the weather decks after the passage of the base surge (in the first few minutes). Shielding Factors. One theoretical method described for calculating shielding factors is rather cumbersome and has not been proven entirely 17-88